T he goal of total knee arthroplasty is to relieve pain and to improve function by creating a knee with adequate range of motion as well as osseous and ligamentous sta- bility. Axial alignment is achieved with resections of the distal femur and proximal tibia. The tibial cut, with the aid of either intramedullary or extramedullary alignment guides, is generally made perpendicular to its long axis. A perpendicular cut is preferred because it is easier to repro- duce and, when performed properly, helps to recreate the mechanical axis of the limb and thus improve the clinical outcome.1–4 Axial alignment of the femur is generally made by resecting the distal femur in 5 to 7 degrees of valgus. Rotational alignment of the femur is achieved with the anterior and posterior distal femoral resections. The mechanics of the patellofemoral joint are heavily depend- ent on this rotational alignment. Improper rotational alignment may cause patellofemoral problems or gross changes in the foot progression angle during the gait cycle.
This chapter addresses the various methods used to achieve proper axial and rotational alignment of the femur in total knee arthroplasty. The influence of femoral alignment on patellofemoral mechanics and how it relates to achieving balanced flexion and extension gaps is also discussed. Particular attention is given to the current technique for achieving proper alignment in the revision setting.
ANATOMY
A tremendous amount of variation occurs in normal limb alignment. Static alignment is affected by height, weight, and bony morphology. Knee kinematics are influenced by the degenerative changes found in arthritic knees. The geometry of the human femur has been well described,
5and several studies examine the specific sizes and shapes of the femur.
6,7In the coronal plane, the anatomic axis is defined as a line drawn down the centers of the femur and tibia (Figure 11-1). On average, this creates an angle of 5 to 7 degrees of valgus at the knee joint. The tibiofemoral angle results from a combination of the varus tilt of the tibial plateau (3 degrees) and the valgus alignment of the femoral condyles, on average 7 degrees.
8The mechanical
axis is defined as a line drawn from the center of thefemoral head, through the center of the knee, and ending in the center of the ankle joint. In general, the mechani- cal axis lies 3 degrees off the vertical axis.
The flexion axis of rotation of the knee is thought to transect a line drawn between the medial and lateral epi- condyles at the origins of the medial and collateral liga- ments. This axis should lie transverse to the long axis of the tibia. At 90 degrees of flexion, the medial condyle extends 1–6mm more posterior than the lateral condyle
9(Figure 11-2). This axis undoubtedly has wide variation, and the amount of the condyles that fall below the transepicondylar axis varies as well.
BIOMECHANICS
The lower extremity goes through 2 stages during the gait cycle. It bears weight in the stance phase and is advanced in the swing phase. Stance phase can be divided into a period of double-limb support followed by a time of single-limb support. The single-limb support segment is further divided into multiple parts: heel strike, foot flat, heel off, and toe off. The contralateral foot enters heel strike shortly after the initial foot passes through heel rise.
Stance phase comprises 62% of the gait cycle while swing phase accounts for 38%.
10117
Femoral Alignment
James Huddleston, Reuben Gobezie, and Harry Rubash
In stance phase of the gait cycle, the medial compart- ment of the knee experiences approximately 60% to 70%
of the weightbearing forces in a lower extremity with 7 degrees of anatomic valgus or a neutral mechanical axis.
Any perturbation in the alignment will likely lead to changes in this distribution, and even small changes may predispose the joint to degenerative arthritis.
11–14Estab- lishing the correct axial and rotational alignment during total knee arthroplasty should serve to reproduce, as closely as possible, the normal distribution of forces seen across the knee joint during gait. This in turn should lead to an overall better clinical result and improve the sur- vivorship of the components.
4It has been shown that even a 5-degree axial malalignment can change the load seen across the knee joint by up to 40%.
15This work was supported by the study of Ritter et al., who concluded that
early failures in total knee arthroplasty were correlated with tibial varus of greater than 5 degrees.
16Further, Berger and Rubash, in comparing 30 patients with isolated patellofemoral complications after total knee arthroplasty to 20 patients with well-functioning total knee arthoplasties, found that patellofemoral complica- tions were directly correlated with combined internal rotation of the femur and tibia.
2They noted that internal rotation of 1 to 4 degrees produced lateral tracking and patellar tilt. Patellar subluxation was seen with 3 to 8 degrees of internal rotation. As the internal rotation increased to 7 to 17 degrees, they reported patellar dislo- cation and early patellar component failure.
AXIAL ALIGNMENT
The mechanical axis of the lower extremity must be restored to neutral for a revision total knee arthroplasty to be successful. Most surgeons will perform the distal femoral resection by aligning it in 5 to 7 degrees of valgus.
17,18It is commonly believed that the tibiofemoral angle should be restored to 6 degrees (+/-1 to 2 degrees)
Figure 11-1. The LE axes. (From Pollice, Lotke, Lonner,9by per-mission of Lippincott Williams & Wilkins, 2003.)
Figure 11-2. Transepicondylar axis. (From Pollice, Lotke, Lonner,9by permission of Lippincott Williams & Wilkins, 2003.)
of valgus. Despite the average 3-degree varus angulation of the native tibial plateau, most surgeons prefer a tibial cut that is perpendicular to the long axis of the tibia. It is important to realize that these numbers may vary slightly depending on such variables as preoperative alignment, collateral ligament integrity, and obesity.
Historically, there are 2 methods for cutting the distal femur and proximal tibia. The tensioning or gap tech- nique relies on an initial transverse tibial resection to assist in achieving rectangular flexion and extension gaps.
9It cannot be overemphasized that a cut perpendi- cular to the long axis of the tibia is crucial for this tech- nique to be successful. The dimensions of the flexion and extension gaps can only be assessed properly once all osteophytes are removed and all ligaments are balanced before tensioning (Figure 11-3).
In the measured resection technique, the surgeon attempts to restore proper alignment by replacing what has been removed by arthritis with exactly the same amount of prosthetic material. When using this method, the femoral resections (distal, anterior, and posterior)
should reflect the thickness of the condylar surfaces of the prosthesis to be implanted. On the tibial side, if 12mm of tibial plateau is resected, then the thickness of the tibial implant (tray and insert if using a modular tibia) should be equal to 12mm (Figure 11-4).
Both extramedullary and intramedullary guides are available to assist the surgeon in cutting the distal femur in 5 to 7 degrees of valgus. It has been shown in multiple series that intramedullary (IM) guides improve the accu- racy of the distal femur resection. In a review of 201 knee arthroplasties in which a standard IM guide was used, Teter et al. used radiographs to show that distal femoral alignment was considered to be accurate 92% of the time.
19They identified femoral bowing and capacious femoral intramedullary canals as risk factors for inaccu- rate distal femoral alignment. The largest series compar- ing the 2 methods involved 200 consecutive total knee replacements, in which extramedullary guides were used in 75 cases and intramedullary guides were used in 125 cases. The postoperative distal femoral alignment was defined as “acceptable” if it fell between 4 and 10 degrees of valgus. They reported that 72% of the extramedullary group versus 86% of the intramedullary group had acceptable alignment. Further, they found that joint line
Figure 11-3. (A and B) Tensioners. (From Insall, Scott,23by per-mission of Churchill Livingstone.) Figure 11-4. Measured resection technique. (From Pollice, Lotke, Lonner,9by permission of Lippincott Williams & Wilkins, 2003.)
orientation was outside of the “normal” range twice as frequently in the extramedullary guide group.
20Based on these findings, most surgeons elect to use an intra- medullary femoral guide when performing total knee arthroplasty. In the revision setting, the use of intramedullary rods is particularly helpful to assist the surgeon in dealing with bone loss and distorted anatomy.
To use an intramedullary alignment guide, the surgeon begins by establishing an entry point located just anterior to the origin of the posterior cruciate ligament.
Flexing the knee facilitates this process. It is advisable in the revision setting to obtain full-length anteroposterior and lateral weightbearing radiographs of the lower extremity. This allows the surgeon to determine the pre- operative axial alignment and to assess the morphology of the femoral intramedullary canal. This is particularly
important in cases of posttraumatic arthritis after femur fracture. The drill hole starting point is usually slightly medial to the center of the intercondylar notch. Place- ment of the guide too medially or too laterally results in cuts that are in excessive varus or valgus, respectively.
Most current knee systems offer cutting jigs that allow the distal femur to be cut in 4 to 7 degrees of valgus alignment.
Fat emboli syndrome is a concern with the use of intramedullary guides. Two techniques have helped to diminish its incidence. Overdrilling of the starting point allows for the release of the intramedullary contents, which diminish intramedullary pressures when the rod is inserted.
21Further, overdrilling should allow for the rod to engage in diaphyseal bone. Flutes in the guide rod have also been shown to decrease intramedullary pressures and to reduce the incidence of fat embolism.
21,22The tensioning technique, as described originally by Insall, can also be used to perform the distal femoral resection.
23This technique relies on a tibial cut that is perpendicular to the long axis of the tibial shaft. Before making any bony cuts, all osteophytes should be removed and soft tissues should be balanced. The extension space is then created under tension by cutting the distal femur parallel to the cut tibial plateau; this ensures a rectangu- lar extension space. The knee is then flexed to 90 degrees, and the tensioners are re-inserted (Figure 11-5). The pos- terior femoral cut is then performed parallel to the tibial plateau. In the primary total knee arthroplasty of a varus knee, proper external rotation can usually be achieved by resecting more off the medial posterior condyle than the lateral posterior condyle. This may not always be the case with revision total knee arthroplasty, as the posterior femoral condyles, if present at all, are likely to be severely distorted (Figure 11-6).
Figure 11-5. Tensioners in laboratory.
Figure 11-6. (A and B) Intraoperative technique. (From Insall, Scott,23by permission of Churchill Livingstone.)
Figure 11-7. Finding the starting hole for the femoral reamer.
(From Rubash H, et al. CCK Technique Guide. In NexGen LCCK Revision, 2001. Courtesy of Zimmer, Inc., Warsaw, IN.)
Figure 11-8. The medullary canal is then reamed larger until cortical contact is made. (From Rubash H, et al. CCK Technique Guide. In NexGen LCCK Revision, 2001. Courtesy of Zimmer, Inc., Warsaw, IN.)
Figure 11-9. Distal femoral cut. (From Rubash H, et al. CCK Technique Guide. In NexGen LCCK Revision, 2001. Courtesy of Zimmer, Inc., Warsaw, IN.)
Revision systems with cutting slots provide a third alternative for achieving proper distal femoral alignment.
The surgeon begins by locating the starting point for the femoral reamer. Some systems offer an IM hole locator to assist in finding this position. This device has an outrig- ger that lies flat on the anterior cortex of the femur and parallel to its anatomic axis (Figure 11-7). Once the start- ing point has been chosen, the starter reamer is then inserted into the medullary canal. Eccentric placement of the intramedullary guide can be avoided by reaming par- allel to the shaft of the femur in both the anteroposterior and medial-lateral planes. Once the canal has been located, the starting hole is enlarged with a step drill. The canal is then reamed progressively larger, generally by hand, until cortical contact is made (Figure 11-8). We stop when cortical engagement occurs. Proper preoperative
planning is crucial in estimating the size of the last reamer to be used.
The valgus angle of the distal femur is then checked by inserting a straight stem into the intramedullary canal.
A standard revision cutting block is then attached to the stem. If this device sits flush on the distal femur, then, for most systems, 5 to 7 degrees of valgus alignment exists between the anatomic axis and the distal cut. If the device does not sit flush, then the distal femur must be recut.
Before doing so, the surgeon should check that the proper side (right vs. left) has been selected.
Recutting of the femur begins by attaching a stem extension onto the distal femur revision cutting block.
Once this has been impacted, a distal femoral cutting
guide is attached to the extension (Figure 11-9). The distal
femoral cutting guide is then stabilized with 2 headless
pins. The intramedullary guide is then removed with an
extractor. At this point, the position of the joint line can
be adjusted by using the +2, -2, +4, and -4 holes that are
found on most distal femur cutting guides. These mark-
ings correspond to millimeters of bone that will be
removed with the resection. The final joint line should be
approximately 2.0 to 2.5cm distal to the epicondyles. An
oscillating saw is then used to make the distal femoral cut.
The femur must then be evaluated for the proper size.
Again, preoperative templating will ensure an accurate estimate as to the final size of the femoral component to be used. Many systems come with sizing templates that can be attached to the intramedullary alignment guide.
These should serve as an estimate only, as the final size will ultimately be selected when one balances the flexion and extension gaps.
The last step is to establish proper femoral rotation and component placement. To do so, the surgeon attaches a femoral base-guide flange to the cutting block. The proper right-left indication must be selected. The flange is then secured to the cutting block, and the device is reinserted over the intramedullary alignment guide. The cutting block should be flush with the distal femur, and the flange should rest on the anterior femoral cortex. An alignment guide is then attached to the posterior edge of the cutting block. We prefer to orient this guide parallel to the surgical epicondylar axis
24(Figure 11-10). If the cutting block is not properly positioned on the distal femur, then an offset femoral stem can be used. This allows for adjustment of the cutting block several mil- limeters in both the anteroposterior and medial-lateral planes. Once the final position has been selected, 2 head- less pins are used to secure the device to the distal femur.
The intramedullary alignment guide is then removed, and the proper anterior and posterior femoral cuts can be made through the corresponding slots (Figure 11-11).
Any bony defects that remain can be addressed with the use of augments. After the size and location of the augment to be used has been determined, the surgeon prepares the femur by cutting through the corresponding augment cutting slots on the femoral cutting guide (Figure 11-12).
Figure 11-10. Rotational alignment guide. (From Rubash H, et al. CCK Technique Guide. In NexGen LCCK Revision, 2001. Cour- tesy of Zimmer, Inc., Warsaw, IN.)
Figure 11-11. (A–C) Setting rotational alignment. (From Rubash H, et al. CCK Technique Guide. In NexGen LCCK Revision, 2001. Courtesy of Zimmer, Inc., Warsaw, IN.)
in total knee arthroplasty today, affecting 4% to 41% of total knee arthroplasties in which the patella was resur- faced.
26,27Up to 45% of all revision total knee arthroplas- ties and 30% to 41% of re-revisions are related to the patellofemoral joint.
1,28These problems include poor tracking, subluxation, anterior knee pain, patellar clunk, and accelerated patellofemoral component wear.
2,3,24,29,30Despite the technological advances afforded by the cur- rent generation of total knee prostheses, patellofemoral complications continue to plague surgeons.
2,31–34In general, 2 methods are used to achieve proper femoral rotational alignment. The first method involves the use of tensioners. The second method relies on bony landmarks. The literature is filled with numerous sup- porters and detractors of the various methods. The revision surgeon must be familiar with the different techniques, as the distortion of anatomy and the bone loss that often accompany revision arthroplasty may not allow the surgeon to use any one particular reference.
In the classic method, the knee is initially balanced in full extension. It is then flexed to 90 degrees, and a cut perpendicular to the long axis of the tibia is performed.
The knee is then tensed in 90 degrees of flexion, and anteroposterior resections, parallel to the tibial plateau, are performed on the femur. This should produce a rec- tangular flexion space, thus assuring proper rotational orientation of the femur relative to the patella and the tibia.
35It is important to realize that this technique may be difficult in cases of substantial preexisting ligamentous imbalance.
Hungerford and Krackow proposed in 1985 that equal amounts of bone must be resected from the medial and lateral posterior femoral condyles
36(Figure 11-13). When unequal amounts of bone are resected off the tibial plateau (as is usually the case in varus osteoarthritis), the femoral component will be internally rotated, if equal amounts of bone are then taken from the medial and lateral posterior femoral condyles. The increased Q-angle likely causes patellar maltracking with subsequent eccen- tric polyethylene wear, subluxation, or even dislocation of the patellofemoral joint. To prevent this problem in the majority of knees, the surgeon must resect more poste- rior condyle from the medial side than from the lateral side. The damage to the posterior femoral condyles from osteolysis or during removal of the primary component will likely render this technique impractical in revision total knee arthroplasty.
The clinical epicondylar axis is one bony landmark that may be used to ensure proper femoral rotation. In 1987 Yoshioki et al. defined the clinical epicondylar axis as the line connecting the lateral epicondylar prominence and the most prominent aspect of the medial epi- condyle.
37Their group also described the condylar twist
ROTATIONAL ALIGNMENT
It has been shown that slight external rotation of the femoral component helps to optimize patellar tracking.
Optimal patellofemoral kinematics help the surgeon to avoid the many pitfalls that may arise from the patellofemoral articulation.
3,25The rotation of the femoral component is critical in determining the surgeon’s ability to achieve a rectangular flexion space.
Problems with the patellofemoral joint are among the
most common postoperative complications encountered
Figure 11-12. (A and B) The femur is prepared by cutting through the corresponding augment cutting slots on the femoral cutting guide. (From Rubash H, et al. CCK Technique Guide. In NexGen LCCK Revision, 2001. Courtesy of Zimmer, Inc., Warsaw, IN.)Figure 11-13. (A and B) Posterior condyle, equal resections and appropriate resections. (From Krackow KA. The Technique of Total Knee Arthroplasty. St. Louis: Mosby; 1990:131.)
Lateral Lateral Epicondylar Prominence
Condylar Twist Angle
Medial
A Prominence on the Medial Epicondylye
Clinical Epicondylar Axis Posterior Condylar
Line
Figure 11-14. Condylar twist angle. (From Berger, Rubash, Seel, et al.24by permission of Clin Orthop.)
A
B
Figure 11-15. (A and B) MCL origin. (Adapted from Berger, Rubash, Seel, et al.24by permission of Clin Orthop.)
angle as the angle subtended by the posterior condylar line and the clinical epicondylar axis (Figure 11-14). The medial prominent point can be palpated through the skin and soft tissues and is located on the crescent ridge that is the point of attachment for the superficial fibers of the medial collateral ligament (Figure 11-15). Many other current total knee systems use the posterior condylar line as their reference point for determining rotational align- ment. The jigs are usually based on a pre-fixed 3 degrees of external rotation off the line drawn between the pos- terior condyles (posterior condylar line) (Figure 11-16).
Again, this point of reference is not always available when
revising a total knee arthroplasty. Further, this technique
may be unreliable with the cartilage wear and bony
defects that are present with arthritis.
38Whiteside’s line is another bony landmark that may assist the surgeon in determining rotation of the femoral component. Described by Whiteside and Arima in 1995, this line runs in the deepest part of the trochlear groove and should be perpendicular to the epicondylar axis
39(Figure 11-17). It is useful as an intraoperative check to ensure proper orientation of the femoral cutting block.
Unfortunately, patellofemoral arthritis may obscure this reference. Further, the anterior and posterior femoral cuts from the index procedure may make it difficult to use this technique in revision arthroplasty.
It is our feeling that the surgical epicondylar axis pro- vides both an important secondary anatomic reference in primary total knee arthroplasty as well as a useful primary anatomic landmark that can be used when the posterior condylar surfaces are not available to accurately gauge rotation of the femoral component. Berger, Rubash, et al.
have defined the surgical epicondylar axis as a line drawn between the lateral epicondylar prominence and the medial sulcus of the medial epicondyle (Figures 11-18 and 11-19). The medial sulcus may be difficult to find intraoperatively. If this is the case, the authors advocate removing any superficial soft tissues and then using a sur- gical marker to define the entire medial epicondyle. The medial sulcus can be found as a depression in the center of the prominence. It is from the medial sulcus that the deep fibers of the medial collateral ligament take origin.
The superficial medial collateral ligament is the fanlike
Figure 11-16. ER off posterior condylar line. (From Callaghan J, Rosenberg A, Rubash H, Simonian P, Wickiewicz T, eds. The Adult Knee. By permission of Lippincott Williams & Wilkins, 2003.)A
Figure 11-17. (A and B) Whiteside’s line. (From Callaghan J, Rosenberg A, Rubash H, Simonian P, and Wickiewicz T, eds. The Adult Knee. By permission of Lippincott Williams & Wilkins, 2003.)
Lateral Epicondylar Prominence
Posterior Condylar Angle
Posterior Condylar Line
Surgical Epicondylar
Axis Medial Sulcus
Figure 11-18. Line drawing of the surgical epicondylar axis.
(From Berger, Rubash, Seel, et al.24by permission of Clin Orthop.)
insertion that overlies the deep fibers. Once identified, the anterior and posterior femoral resections should be per- formed parallel to this axis.
The anterior trochlear groove is also a useful intraop- erative reference to assist in determining the correct amount of external rotation of the femoral component.
It has been well described that in a normal femur, the lateral side is more prominent than the medial side. When the surgeon performs the anterior femoral cut, more of the lateral side should be resected than the medial side.
When the cut is performed correctly, more cancellous
bone should be visible laterally (Figure 11-20). This sign is commonly referred to as the Insall boot. If the resection is made in neutral or in internal rotation, one would see more cancellous bone on the medial side. If this is the case, we recommend reassessing and performing a cut parallel to the surgical epicondylar axis.
In 1999, Olcott and Scott compared the efficacy of these various reference axes.
40They evaluated 100 con- secutive primary total knee arthroplasties in 81 patients performed for both osteoarthritis (93 knees) and rheumatoid arthritis (7 knees) by one surgeon (R.D.S.).
The femoral alignment necessary to create a balanced flexion gap was determined and compared with White- side’s line, the transepicondylar axis, and a line in 3 degrees of external rotation off the posterior femoral condyles. They found that the transepicondylar axis most consistently recreated a balanced flexion space. The 3 degrees off the posterior condyles was least consistent, especially in valgus knees.
Katz et al. found that the tension technique, as described initially by Insall, was the most reliable in deter- mining the correct femoral rotation.
41Their group also reported that the transepicondylar axis (both clinical and surgical) had the greatest variation. Agaki et al. in 2001 reached conclusions similar to those reached by Olcott and Scott with regard to the posterior condylar line.
42They used computed tomography to evaluate the poste- rior condylar line, the anteroposterior line, and the transepicondylar axes (surgical and clinical) in 111 symp- tomatic arthritic knees. The tibiofemoral and distal femoral valgus angles were then compared with the pre- viously mentioned reference angles. Their group found that the posterior condylar angle became unreliable when the tibiofemoral valgus angle exceeded 9 degrees. They
were unable to locate the medial sulcus of the surgical epi- condylar axis in 25% of the cases. The authors concluded that the anteroposterior axis was more reliable in valgus knees, and they advocated the use of computed tomogra- phy for knees with severe valgus deformity.
The medial/lateral placement of the femoral compo- nent should not be overlooked, as it may influence patel- lar tracking as well.
25In most cases, the mediolateral width of the femoral component occupies most of the bony surface. However, if some cancellous bone remains visible, we recommend lateralizing the component. The femoral component should be adjusted until the lateral edge of the prosthesis bisects the cut lateral surface of the femur (Figure 11-21). This effectively lateralizes the trochlear groove and thus optimizes patellar tracking.
Figure 11-19. Photograph of the surgical epicondylar axis.
(From Callaghan J, Rosenberg A, Rubash H, Simonian P, Wickiewicz T, eds. The Adult Knee. By permissions of Lippincott Williams & Wilkins, 2003.)
Figure 11-20. Insall boot. (From Callaghan J, Rosenberg A, Rubash H, Simonian P, Wickiewicz T, eds. The Adult Knee. By per- missions of Lippincott Williams & Wilkins, 2003.)
Anterior
Posterior
Lateral Medial
Cut distal femur
Femoral prosthesis
Figure 11-21. Mediolateral placement of the femoral component. The femoral component should be adjusted until the lateral edge of the prosthesis bisects the cut lateral surface of the femur.
Conversely, medialization of the prosthesis causes lateral- ization of the patella relative to the trochlear groove (Figure 11-22). This should be avoided, as it is likely to have a negative impact on patellar tracking.
SUMMARY
The alignment of the femoral component is vital to the success of any total knee arthroplasty. Alterations in the normal alignment likely lead to decreased component survivorship and poor clinical outcomes. Revision total knee arthroplasty poses particular challenges to the surgeon with regard to femoral alignment.
43,44At the Massachusetts General Hospital, we believe that proper axial alignment can be achieved with the use of an intramedullary alignment guide. This should allow for the distal femur to be cut reliably in 5 to 7 degrees of valgus.
Proper rotational alignment is achieved by making the anterior and posterior femoral resections parallel to the surgical epicondylar axis. It is our feeling that the surgi- cal epicondylar axis is a reliable landmark that can be used in even the most difficult revision cases. Restoration of the native axial and rotational alignment of the femur improves the chances of achieving a successful and durable revision total knee arthroplasty.
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